There are many different kinds of gas heaters and other gas appliances which utilize an electric fan motor for providing air to a combustion chamber in an appropriate amount such that the fuel/air mixture is at the desired ratio to produce a clean-burning efficient blue flame. Secondarily, the fan may also provide air flow around and over the combustion chamber which is heated thereby, as desired These appliances, including especially space heaters, typically include a gas valve which has a manually operated opening plunger which is mechanically biased by a return spring into the closed position, and an electromagnetic solenoid coil which, when energized, overcomes the force of the spring to hold open the gas valve to permit gas flow therethrough. To turn on the gas appliance, or heater, the gas valve plunger must be manually pushed and held open to permit gas flow therethrough as an operator manually ignites the flame with a piezoelectric igniter, for example. After ignition, the plunger must be held down for some time period, typically 30 seconds, until a thermocouple located in the combustion chamber is heated to a sufficient temperature in order to generate a sufficient current to hold the solenoid coil energized or latched in and thereby hold the gas valve open. As the thermocouple achieves this minimum temperature, the operator may release the plunger on the gas valve which will then remain in the open position permitting continued automatic operation of the heater. In order to turn the heater off, the proper sequence of operation requires that a main valve in the gas line be turned off which permits the residual gas in the line to flow through the solenoid operated gas valve and continue to burn until the gas is exhausted therefrom. As the flame self-extinguishes, the thermocouple temperature drops thereby reducing the amount of current produced thereby until the manual return spring on the gas valve overcomes the holding power of the solenoid coil and thereby manually closes.
Presuming the correct sequence of operation is followed, the gas line is self-exhausting and no unsafe or hazardous conditions are experienced. However, there are opportunities for improper operation, or for equipment difficulties to cause an unsafe condition. For example, if the operator disconnects the electrical power to the fan motor before shutting off the solenoid operated gas valve, or if the power to the fan motor is interrupted for any reason such as through an overload condition, accidentally turning the fan motor off by the operator, or inadvertently tripping over or otherwise pulling the power plug from its receptacle, the fan motor will cease to provide appropriate air for combustion to the combustion chamber but the gas will continue to flow through the solenoid operated gas valve. This fuel/air mixture typically will produce a significantly sized but weak, yellowish, inefficient flame to create a possible fire and fume hazard. This may typically continue for 30-40 seconds as the incomplete combustion and reduced temperature flame allows the cooling of the heat sensing thermocouple to a temperature where insufficient current and electrical power is generated to the solenoid holding coil to maintain the gas valve in an open position. After the thermocouple cools, as previously explained, the mechanical return spring finally overcomes the solenoid coil to close the gas valve and thereby extinguish the flame.
In the prior art, some gas appliances, including especially space heaters, are supplied with a normally open relay as an attempt at eliminating this unsafe condition. In this prior art solution, the relay coil winding is connected in parallel with the fan motor windings and its normally open switch contacts are connected in series with the thermocouple. Therefore, if power is interrupted to the fan motor, the relay coil de-energizes which opens its normally open contact to thereby electrically disconnect the thermocouple from the solenoid coil which will, ideally, permit the mechanical return spring to close the gas valve and thereby extinguish the flame. However, in application, these relays suffer several drawbacks. As the potential generated by the typical thermocouple utilized in these space heaters ranges between 7 and 20 millivolts, any appreciable increase in the contact resistance of the relay's normally open contact, as might be caused by contact oxidation or the contamination often experienced in these applications, prevents the flow of current to the gas valve even after the thermocouple reaches its appropriate temperature In this condition, the potential generated by the thermocouple will never be sufficient enough to hold the solenoid coil in an energized condition to overcome the mechanical return spring for the gas valve. Thus, an operator will remove his finger from the gas valve after the appropriate 30 second interval but the gas valve will turn off thereby preventing operation of the heater entirely In frustration, an operator may decide to short around the relay contacts in order to get the heater to work which will completely eliminate any protection provided by the relay.
In still another failure mode, the relay contact may become permanently closed in the event of a failure in the relay, such as through breakage of the armature return spring. The relay contacts may also become permanently bridged through the presence of metallic contamination. In either event, with the relay contacts remaining closed, the protection presumed to be afforded by the relay is not provided and an operator may be unaware of this unsafe condition. Additionally, an electromagnetic relay may be momentarily jarred open with any appropriate shock or impact which will result in a nuisance flame-out of the heater. Still another potential failure includes the chance for improper electrical connection to the fan motor which could result in connecting the relay directly to the fan motor coils and not in circuit with the thermo-protector. In this event, the thermoprotector could open to turn off the fan motor but yet the relay would remain energized to thereby create the conditions for a poorly burning flame as described above.
In order to solve these and other problems in the prior art, and to provide a safe and certain turnoff of a solenoid operated gas valve in a gas appliance, such as a space heater, the inventors herein have succeeded in designing and developing an electronic gas valve control circuit which may be conveniently mounted on a PC board and provided inside the fan motor itself such that a pair of leads may be conveniently connected in circuit with the solenoid coil for the gas valve. The electronic control circuit of the present invention includes a half wave rectifier circuit and capacitor for building up a DC potential, and a solid state switching circuit for sensing the loss of AC power to the fan motor and for applying the voltage from the charged capacitor to the solenoid coil in a reversed polarity which immediately turns off the solenoid coil to permit the immediate closing of the gas valve by the mechanical return spring. Several embodiments of this solid state circuit are disclosed, all of which are effective in applying an opposing flux to the solenoid coil instead of merely interrupting the current to the solenoid coil as in the prior art. In this manner, a safe and reliable safety circuit achieves immediate turn-off of the gas valve, eliminates the "hard starting", nuisance flame-out, and various other failures experienced by the safety relay of the prior art, and virtually eliminates any potential for misconnection by an operator or manufacturer which might otherwise defeat the safety feature expected by an operator. The present invention is relatively inexpensive, may be self-contained in the fan motor as mentioned above, and reliably eliminates the hazardous conditions experienced due to a gas valve which remains open while the thermocouple cools upon loss of an adequate air supply to support an adequate combustion in the combustion chamber
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow